Carburized and hardened alloy steel gears are widely used for vehicle power trains in order to obtain a sufficient resistance to surface pitting and high bending loads, and thereby a generally desirable service life. However, the heat treating and processing of such gears takes a long time, uses a considerable amount of energy and, accordingly, the gears are expensive. Drastic quenching of the gears is also often required, which results in considerable distortion. Moreover, even with such extensive processing, the microstructure of the gears is inhomogeneous and the gears lack sufficient case toughness at the desired high hardness levels. Likewise, nitrided alloy steel gears are relatively brittle at relatively high hardness levels, for example, above a magnitude of about 58 on the Rockwell C hardness scale (Rc58), and do not exhibit a relatively uniform metallographic structure.
The properties of some bainitic alloy steels are very desirable as a substitute for the above-mentioned martensitic steels. For example, in high carbon steels low temperature bainite is more ductile than martensite at the same hardness level. But most prior art bainitic alloy steels have utilized controlled amounts of potentially critical and/or expensive materials such as chromium and nickel. Exemplary of the art in this area are the following U.S. Pat. Nos.: 3,418,178 to S. A. Kulin et al on Dec. 24, 1968; 3,303,061 to J. E. Wilson on Feb. 7, 1967; 2,128,621 to B. R. Queneau on Aug. 30, 1938; 3,298,827 to C. F. Jatczak on Jan. 17, 1967; 3,348,981 to S. Goda et al on Oct. 24, 1967; 3,366,471 to M. Hill et al on Jan. 30, 1968; 3,418,178 to S. A. Kulin et al on Dec. 24, 1968; 3,528,088 to H. Seghezzi et al on Sept. 8, 1970; and 3,907,614 to B. L. Bramfitt et al on Sept. 23, 1975. A considerable portion of the available carbon is tied up by the critical alloy in the aforementioned patent references so that a relatively coarse crystallographic texture is formed that is detrimental to the toughness and hardness of the final article. Another disadvantage is that when such critical alloys are utilized, the austenite transformation temperature is generally raised, thereby requiring increased amounts of energy and adding to the processing costs.
U.S. Pat. No. 1,924,099 issued to E. C. Bain et al on Aug. 29, 1933 describes a process known as austempering. Such process involves the steps of: (a) heating a steel article above an upper critical temperature to assure a change in the morphology of the article to substantially 100% austenite; (b) quenching the article below approximately 540.degree. C. (1000.degree. F.), but above the temperature of martensite formation or the so-called martensite start (M.sub.s) line; and (c) holding the steel article at such an intermediate temperature for a preselected period of time sufficient to convert the morphology of the article to a form other than 100% martensite. While time-temperature-transformation (TTT) diagrams are known on various prior art steels, the alloy compositions and austempering processes resorted to have suffered two general deficiencies. Firstly, the quenching step has involved cooling at a rate such that the transforming start (T.sub.s) curve of the alloy has been crossed and undesirable upper transformation products such as proeutectoid ferrite/carbide, pearlite, and upper bainite formed. This is due to a major degree to the relatively critical time period of the alloy to the usual nose portion of the transformation start curve. The undesirable crossover of such nose portion results in a loss of toughness and hardness. Secondly, the heat treat holding times sufficient to obtain substantially complete transformation have been too long, for example, five hours or more. For general commercial applications, such an extended holding period is substantially impractical and represents a considerable waste of energy and time.
Still another factor of major importance is that as the body of the article to be produced increases in thickness, its cooling rate decreases. As a consequence, although thin nails or the like having an appreciable proportion of lower temperature bainite may have been made because such articles could be rapidly cooled in a practical manner, it is believed that the prior art TTT diagrams of the materials of such articles would not permit the practical production of a substantially complete lower temperature bainite morphology when the section thickness is increased to about 12 mm (1/2"), for example. Thus, the composition taught by U.S. Pat. No. 3,528,088 to H. Seghezzi et al on Sept. 8, 1970 is not desirable for an article thicker than about 12 mm (1/2") because the austempered morphology of the article would vary nonuniformly across its cross section away from substantially complete lower bainite, and the hardness would undesirably drop below a magnitude level of about 55 to 58 on the Rockwell C scale. The anchoring device of the Seghezzi patent also undesirably utilizes the potentially critical element chromium. U.S. Pat. No. 3,196,052 to K. G. Hann on June 28, 1961 is representative of another alloy steel for a thin wire that has substantially the same problems.
In some instances, such as in U.S. Pat. No. 2,814,580 to H. D. Hoover on Nov. 26, 1957, processing of certain steel alloys has been accomplished at a holding temperature which is above the level of lower bainite formation. In other instances, although a lower holding temperature may have been established, austenite, pearlite, upper bainite or martensite have been undesirably present to an excessive proportion in comparison with the lower temperature bainite, for example, above an individual or collective proportion of about 10 Vol.%.
The present invention is directed to overcoming one or more of the problems as set forth above.